flies on: “There are no flies on him,” is a slang phrase not used by persons accustomed to refined diction.
flock: A word sometimes misapplied. Do not say “a flock of girls;” say, rather, “a bevy of girls” and “a flock of sheep.” Flock is correctly applied to a company or collection of small animals as sheep, goats, rabbits, or birds.
flop is an inelegant word used sometimes to denote change of attitude on a subject. Do not say “He flopped over to the other side”; say, rather, “He went over....”
flub-dub: A slang term used to designate a literary work that is worthless.
flummux: A vulgarism sometimes used for “perplex” or “disconcert.”
fly off the handle: A colloquial phrase meaning to “lose one’s self control” as from anger.
folks: The modern colloquial plural use of this term is not to be recommended. The word is properly used, both in singular and plural form, as folk, its correct signification being “people, collectively or distributively.”
foment, ferment: Exercise care in the use of these words. Foment is to bathe with warm or medicated lotions; ferment, to cause chemical decomposition in. Both words are also used figuratively.
fondling, foundling: Discriminate carefully between these words. A fondling is a person fondled or caressed; a foundling is a deserted infant whose parents are unknown.
fooling: The use of the word in the sense of “deceiving” has been condemned by certain writers as a “very vulgar vulgarism,” but is permissible, having the sanction not only of good literary authority but of modern dictionaries. See Tennyson’s “Gareth and Lynette” (st. 127): “Worse than being fool’d of others is to fool one’s self.”